I think that's just because people couldn't remember which was "slash" and which was "backslash", and in the DOS/Windows world, you were more likely to hear ref

I think that's just because people couldn't remember which was "slash" and which was "backslash", and in the DOS/Windows world, you were more likely to hear references to "backslash". Browsers quickly started silently fixing inappropriate backslashes, so people never had to learn which was which.